1. Field of the Invention
This invention relates to bipolar plate assemblies for fuel cells and particularly to methods for bonding and sealing the component plates together.
2. Description of the Related Art
Fuel cells such as solid polymer electrolyte or proton exchange membrane fuel cells (PEMFCs) electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1 V, a plurality of cells is usually stacked together in series for commercial applications. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields may be formed on the electrochemically inactive surfaces of the flow field plates and thus can distribute coolant evenly throughout the cells while keeping the coolant reliably separated from the reactants.
Bipolar plate assemblies comprising an anode flow field plate and a cathode flow field plate which have been bonded and appropriately sealed together so as to form a sealed coolant flow field between the plates are thus commonly employed in the art. Various transition channels, ports, ducts, and other features involving all three operating fluids (i.e. fuel, oxidant, and coolant) may also appear on the inactive side of these plates. The operating fluids may be provided under significant pressure and thus all the features in the plates have to be sealed appropriately to prevent leaks between the fluids and to the external environment. A further requirement for bipolar plate assemblies is that there is a satisfactory electrical connection between the two plates. This is because the substantial current generated by the fuel cell stack must pass between the two plates.
The plates making up the assembly may optionally be metallic, in which case they are typically welded together so as to appropriately seal all the fluid passages from each other and from the external environment. Additional welds may be provided to enhance the ability of the assembly to carry electrical current, particularly opposite the active areas of the plates. Metallic plates may however be bonded and sealed together using adhesives.
The plates making up the assembly may also optionally be carbonaceous (e.g. formed graphite plates) and such plates are frequently sealed together using elastomeric contact seals with the entire stack being held under a compression load applied by some suitable mechanical means. More recently, bipolar plate assemblies are being prepared using adhesives that are capable of withstanding the challenging fuel cell environment. Commonly, epoxy resin adhesives are employed for this purpose and are applied by screen printing or are otherwise dispensed in a pattern suitable for isolating each desired fluid cavity. Typically such adhesives must undergo a heat curing step.
Using heat-cured adhesives to bond and seal plates together can avoid the complexity and attention required when employing numerous elastomeric seals. However, there are several problems with using such adhesives. For instance, pot life of the adhesive can be short, so any equipment that is directly exposed to the adhesive during the application process must be cleaned periodically with strong solvents to prevent adhesive from curing on the equipment. Further, heat-curing can involve a lengthy, energy intensive thermal cycle. Alternative curing methods can also be problematic. For instance, UV curing is difficult because the adhesion joints are not externally visible. And anaerobic curing can be inhibited by the carbon plate materials themselves (typical adhesive resins require metallic ions present to initiate curing, but such ions are otherwise undesirable in the fuel cell). Further still, fixtures are typically required to accurately align, press, and hold the plates together during the cure cycle. Such fixtures must remain dimensionally stable over multiple thermal cycles, and this can be a challenging requirement. And additionally, the material properties of the adhesive can radically change between the time of application and the time of curing with undesirable consequences (e.g. the viscosity may significantly reduce as the glue is heated and run out of the joint or spread via capillary action across the parts and onto the fixtures used to locate and press the plates during curing).
Microencapsulated adhesives (or pressure activated adhesives) have been suggested for certain purposes in the construction of fuel cells. Generally, such adhesives have been suggested for use in bonding certain components together, but other components are relied on for sealing purposes. As an example, published patent application JP2009099531 discloses use of a micro-capsule type bonding agents for bonding certain components together.
Despite the advances made to date, there remains a need for greater simplification, reliability, and cost reduction in fuel cell assembly processes. This invention fulfills these needs and provides further related advantages.